In document copier machines of the electrophotographic type charged latent images are produced on a photoreceptive material and then developed through the application of a developer mix. Where the photoreceptive material is separate from the copy paper itself, a transfer of the developed image to the copy paper takes place with subsequent fusing of the developed image to the paper. A common type of developer mix currently in use in such machines is comprised of a carrier material, such as a magnetic bead, coated with a colored powdery substance called toner. It is the toner which is attracted to the charged, latent image to develop that image and it is the toner which is then transferred from the latent image to the copy paper (where the copy paper is separate from the photoreceptive material). Finally it is the toner which is then fused to the copy paper to produce the finished copy.
It is apparent from the procedure outlined above that toner is a supply item which must be periodically replenished in the developer mix since the toner is carried out of the machine on the copy paper as a reproduced image. It is also apparent that the concentration of toner particles in the developer mix is significant to good development of the latent image since too light a toner concentration will result in too light a developed image and too heavy a toner concentration will result in too dark a developed image.
Literally hundreds of schemes have been developed for maintaining the concentration of toner in a developer mix. The related patent applications, named above, describe one of the best toner concentration control schemes known to the inventor.
Whatever the method of toner concentration control chosen for use in a particular apparatus there remain other variables which seriously affect copy quality. Basically, the density of the development of a toned solid xerographic image is a function of three variables: (1) toner concentration; (2) the image voltage of the photoconductor; and (3) bias voltage on the developer. As discussed above, there are many schemes for controlling toner density.
In the xerographic process the photoconductor is charged to a uniform level at an elevated voltage. The photoconductor is then subjected to illumination to dissipate the charge on the photoconductive surface. The illumination is generally reflected off the surface of a document to be copied such that the white areas of the document to be copied reflect a large amount of illumination and discharge the photoconductor to a low level, whereas the colored areas reflect a low level of light and consequently leave a relatively high charge on the photoconductor. Shades of grayness discharge the photoconductor to varying charge levels. In that manner the photoconductor is made to bear the latent image of the original document. Thus, the variable named above, "image voltage on the photoconductor," is generic to a so-called "white voltage" representative of the areas on the photoconductor which have been discharged by reflected illumination from a white portion of the document to be copied; a "black voltage" which is produced at the relatively undischarged areas of the photoconductor representative of black portions of the original document to be copied; and various "gray voltages" representative of variously colored or shaded areas of the original document.
Once the charged latent image is produced on the photoconductor the image is then subjected to a development technique wherein a colored powdery material called toner is placed upon the latent image. At the development area a development voltage is applied in order to produce a uniform toner distribution in the solid black and solid colored or gray areas of the latent image. In magnetic-brush type developers this is often accomplished by applying a bias voltage directly to the magnetic brush. Regardless of the type of developer used, a quantity termed the "white vector" can be defined which is the absolute value of the white voltage minus the bias (development) voltage; a "black vector" can be defined which is the absolute value of the black voltage minus the bias voltage; a gray vector can be defined for a particular shade of gray which is the absolute value of the gray voltage minus the bias voltage; and any single color vector which is the absolute value of the single color voltage minus the bias voltage.
The inventor herein notes that two of the three variables defining the density of toned solid xerographic images are contained in the definitions of the white vector, black vector and gray vector, i.e., the voltage on the photoconductor and the bias voltage on the developer. Therefore, the inventor reasoned, if we are able to control toner concentration and pin the vector, i.e., control the value of, for example, the white vector, all of the three major variables which go into the development of a solid xerographic image have been controlled or at least balanced. As a result, repeatable high quality images over the life of a photoconductor can be reasonably assured even though the surface characteristics and electrostatic quality of the photoconductor change with age and use; even though there is a tendency for toner to film the photoconductor with use; and even though Teflon, if used in the system, tends to film the photoconductor. This reasoning is equally applicable to a range of color images, for example, magenta.